Subsurface upgrading of heavy oil via solvent deasphalting has been reported previously under laboratory and field conditions. However, these processes require a relatively high solvent-to-oil ratio (SvOR > 1:1 v/v) to induce subsurface asphaltene precipitation, increase oil production, and upgrade crude oil in situ. In our previous work, lab experiments demonstrated that asphaltene precipitants reduce the SvOR (similar to 30-50 vol %) for subsurface upgrading at initial reservoir conditions and when heat is also applied. In this work, the preparative separations were carried out using benzoyl peroxide (BP), Fe2O3, and NiO nanoparticles as asphaltene precipitants for Venezuelan and Canadian heavy crude oils. Initial experiments showed that BP is the most effective additive, producing an increase of similar to 21 wt % in the asphaltene content for a 2500 mg/kg dosage. Preparative separations at 5:1 vol/wt ratio and 50 degrees C showed that the order of activity as asphaltene precipitants is BP > NiO > Fe2O3. In the presence of nickel- and iron-containing precipitants, most of these metals are found in the asphaltenes indicating that the nanoparticles are acting as nucleation sites. Spectroscopic and mechanistic studies using BP as precipitant suggest a free radical mechanism that involves the thermally initiated homolytic cleavage of BP, follow by abstraction of a hydrogen atom from the asphaltenes or maltenes to produce free radical species. In the termination steps, the latter species react with each other to generate new asphaltene species that are not present in the original crude oils.